Radioisotope techniques have been applied to the study of the kinetics of peritoneal dialysis with the goal of ascertaining peritoneal pore size. Results to date suggest a pore size significantly smaller than anticipated and very likely similar to that of the cellulosic membranes of hemodialyzers. Significant membrane resistance has been demonstrated to the transperitoneal movement of solutes with molecular weights as low as 180 with progressively increasing resistance being demonstrated for solutes in the middle molecular weight range of 323 to 1,400. These data raise the question of whether supposed augmented removal of middle molecular weight uremic solutes by peritoneal dialysis as compared to hemodialysis truly occurs and if it does whether it results primarily from the longer duration of peritoneal dialysis or from the removal of such solutes form pre-existing reservoirs in ascitic fluids.